Relays can be used in protection of electric networks and devices, for instance. Relay protection functions can be implemented as constant-time functions, where the operation time is independent of the value of the characteristic measure, such as current, voltage or frequency. Alternatively, relay protection functions can be inverse-time dependent, when the operation time is inversely dependent on the magnitude of the measured signal.
The inverse-time based algorithm analysis can be based on the assumption that the current or overcurrent exceeding a set current threshold value or undercurrent undershooting a set current threshold value, is constant during the measurement time. This results in a simple analogy between theoretical calculations and implementation. However, in practice, fluctuation and variation in the characteristic measure magnitude level can result in a realized operation time of the device.